70 LIGHT SCIENCE FOR LEISURE HOURS. 



make use of those observations were misled. Hitic 

 illce lacrymce. Hence it is that an undeserved reproach 

 has fallen upon the ' exact science.' 



The amount of the error resulting from the mis- 

 interpretation of the observations made in 1769 was, 

 however, very small indeed, when its true character is 

 considered. It is, indeed, easy to make the error seem 

 enormous. The sun's distance came out some four 

 millions of miles too large, and that seems no trifling 

 error. Then, again, the resulting estimate of the dis- 

 tance of Neptune came out more than a hundred million 

 miles too great ; while even this enormous error was as 

 nothing when compared with that which resulted when 

 the distances of the fixed stars were considered. 



But this is an altogether erroneous mode of esti- 

 mating the effect of the error. It would be as absurd 

 to count up the number of hairs' breadth by which the 

 geographer's estimates of the length and breadth of 

 England may be in error. In all such matters it is 

 relative and not absolute error we have to consider. 

 A microscopist would have made a bad mistake who 

 should over-estimate the length of a fly's proboscis by 

 a single hair's breadth ; but the astronomer had made 

 a wonderfully successful measurement of the sun's 

 distance who deduced it within three or four millions 

 of miles of the true value. For it is readily calculable 

 that the error in the estimated relative bearing of the 

 sun as seen from opposite sides of the earth corresponds 

 to the angle which a hair's breadth subtends when seen 

 from a distance of 125 feet. 



